In the operation of aircraft control surfaces, the loss of driving input may allow external loads to backdrive the actuation system and control surfaces in an uncontrolled manner, sometimes called "fluttering". In order to prevent such uncontrolled conditions, many types of no-back devices have been developed, only a couple of which are shown in U.S. Pat. Nos. 3,599,507, dated Aug. 17, 1971, and 4,173,322, dated Nov. 6, 1979. Such systems as shown in those patents use internal forces, such as spring forces, to force a control surface into a neutral position. It not only is desirable to prevent uncontrolled fluttering of the control surface, but it also, however, would be desirable to permit the external loads (i.e. through the control surface itself) to drive the control surface to a neutral or "faired" position and then lock the control surface in that position.
In situations where a prime mover is used to provide torque to the flight control surface, hydraulic pressure often is used in the prime mover means. It would be desirable to provide an automatic neutral position locking system for the control surface should that hydraulic pressure be lost. Air flow against the control surface will tend to move it toward the neutral position. This invention takes advantage of that tendency by permitting the control surface to move toward the neutral position, but prevents external forces or loads, such as the air flow, from moving the control surface in a direction away from the neutral position. In other words, the control surface itself may be operatively associated within the system to effectively actuate the locking means.